The commonest recording materials used in heat-responsive recording systems are leuco dye materials. However, these heat-sensitive recording materials undergo undesirable changes such as erasure or change of color on handling after recording or upon contact with an adhesive tape, diazo copying paper or the like.
To develop a heat-sensitive recording material free of such disadvantages, much research has recently been undertaken to develop diazo type heat-sensitive recording materials. However, as diazo compounds are inherently unstable, the background of the record tends to be stained under various conditions.
Japanese Patent Application (OPI) No. 201743/82 (corresponding to U.S. patent Ser. No. 552,892) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application open to public inspection") teaches a heat-sensitive recording material comprising microcapsules containing a photopolymerizable vinyl compound, a photopolymerization initiator and one color reactant as the nucleus composition, and a mating color reactant outside the microcapsule disposed on the same side of a support. When such a recording material is heated, the color reactant contained in the core of each microcapsule penetrates through the capsule wall or the other color reactant capable of inducing a color reaction penetrates through the wall into the core of the microcapsule. As a result, a color is produced in either case. Therefore, the color can be selectively developed in the heated areas. Thereafter, the recording material is uniformly exposed to light to polymerize the vinyl compound contained in the core to thereby arrest migration of the image forming dye and to prevent development of color in the background area (i.e., the material is fixed).
Another known system is disclosed in Japanese Patent Application (OPI) Nos. 123086/82 and 125092/82 (corresponding to U.S. Pat. No. 4,411,979), for instance, wherein a light-sensitive heat-sensitive recording material containing a diazo compound, a coupling agent, an alkali generating agent and an auxiliary color developing agent is used for thermal recording, followed by irradiation with light so as to decompose the unreacted diazo compound to terminate the color reaction. However, this recording medium may undergo undesirable discoloration (fog) on the shelf prior to exposure due to gradual precoupling.
To avoid this disadvantage, Japanese Patent Application (OPI) No. 190886/84 (corresponding to U.S. patent Ser. No. 600,267) proposes to incorporate in the core of a microcapsule at least one of the diazo compound, coupling agent and auxiliary color developing agent.
The above-mentioned light-fixable heat-sensitive recording material utilizing microcapsules has the advantages of simplicity of recording device, good shelf life, and stability of the image and background of the record but since at least one of the color reactants is isolated by the microcapsule wall, it has the disadvantage of inadequate thermal color development so that it sometimes fails to give a sufficiently intense color in high speed recording using a brief thermal recording signal, i.e., a signal having a short pulse width.
On the other hand, in the example of Japanese Patent Application (OPI) No. 91438/84, in which the composition of the nucleus material using the acid-base dye must be cured by photopolymerization, and even in the case of a non-photopolymerizable core compositioh (i.e., a core component which does not contain both a vinyl compound and a photopolymerization initiator), the above-mentioned isolation of color reactants by the capsule wall before and after printing input results at times in a decrease in color intensity, though the shelf life and record stability are improved.